Looping

' Basic For..Next (1 to 10)
For i As Integer = 1 To 10
    lstNumbers.Items.Add(i)
Next

' Custom Step: count by 2 (odd numbers)
For i As Integer = 1 To 19 Step 2
    lstOdd.Items.Add(i)       ' 1, 3, 5, 7 ... 19
Next

' Negative Step: count down
For i As Integer = 10 To 1 Step -1
    lblCountdown.Text &= i & " "   ' "10 9 8 7 6 5 4 3 2 1 "
Next

' Sum 1 to N using For..Next
Dim n As Integer
Integer.TryParse(txtN.Text, n)
Dim total = 0
For i As Integer = 1 To n
    total += i
Next
lblSum.Text = $"Sum 1..{n} = {total}"     ' fast closed-form: n*(n+1)/2

' Iterate an array by index
Dim scores() As Integer = {88, 72, 95, 61, 84}
Dim highest = scores(0)
For i As Integer = 1 To scores.Length - 1
    If scores(i) > highest Then highest = scores(i)
Next
lblHighest.Text = $"Highest: {highest}"

' Double Step: 0.0 to 1.0 in steps of 0.1
For t As Double = 0.0 To 1.0 Step 0.1
    lstPercent.Items.Add($"{t:P0}")
Next

' Iterate a string array
Dim fruits() As String = {"Apple", "Banana", "Cherry", "Durian"}
For Each fruit As String In fruits
    lstFruits.Items.Add(fruit)
Next

' Calculate average from a List(Of Integer)
Dim scores As New List(Of Integer) From {88, 72, 95, 61, 84}
Dim total = 0
For Each s As Integer In scores
    total += s
Next
lblAvg.Text = $"Average: {total / scores.Count:F1}"

' Iterate all TextBoxes on the form to clear them
For Each ctrl As Control In Me.Controls
    If TypeOf ctrl Is TextBox Then
        CType(ctrl, TextBox).Clear()
    End If
Next

' Dictionary iteration
Dim capitals As New Dictionary(Of String, String) From {
    {"Malaysia", "Kuala Lumpur"},
    {"Japan",    "Tokyo"},
    {"France",   "Paris"}
}
For Each kvp As KeyValuePair(Of String, String) In capitals
    lstCapitals.Items.Add($"{kvp.Key}: {kvp.Value}")
Next

' Validate all TextBoxes before form submit
Dim allFilled = True
For Each txt As TextBox In {txtName, txtEmail, txtPhone}
    If String.IsNullOrWhiteSpace(txt.Text) Then
        txt.BackColor = Color.LightPink
        allFilled = False
    Else
        txt.BackColor = Color.White
    End If
Next
btnSubmit.Enabled = allFilled

' Basic Do While (count up)
Dim counter = 1
Do While counter <= 10
    lstCount.Items.Add(counter)
    counter += 1             ' MUST advance counter or loop runs forever!
Loop

' Read numbers until sentinel 0
Dim running = True
Dim sumTotal = 0
Do While running
    Dim input = InputBox("Enter a number (0 to stop):")
    Dim n As Integer
    If Integer.TryParse(input, n) AndAlso n <> 0 Then
        sumTotal += n
    Else
        running = False     ' exit condition
    End If
Loop
lblTotal.Text = $"Total: {sumTotal}"

' Collatz sequence: keep halving or tripling until you reach 1
Dim n = 27
Dim steps = 0
Do While n <> 1
    If n Mod 2 = 0 Then
        n = n \ 2           ' even: halve
    Else
        n = 3 * n + 1       ' odd: triple + 1
    End If
    steps += 1
Loop
lblSteps.Text = $"Collatz(27) took {steps} steps"   ' = 111 steps

' Do Until: runs while counter > 0
Dim counter = 10
Do Until counter = 0
    lstCountdown.Items.Add(counter)
    counter -= 1
Loop

' Equivalent Do While version (both correct, Until reads nicer here)
Do While counter > 0
    lstCountdown.Items.Add(counter)
    counter -= 1
Loop

' Keep asking until valid number entered
Dim age As Integer
Dim valid = False
Do Until valid
    Dim input = InputBox("Enter your age (1-120):")
    If Integer.TryParse(input, age) AndAlso age >= 1 AndAlso age <= 120 Then
        valid = True
    Else
        MsgBox("Please enter a number between 1 and 120.")
    End If
Loop
lblAge.Text = $"Your age: {age}"

' Do..Loop While: body runs first, then condition is checked
Dim counter = 1
Do
    lstItems.Items.Add($"Item {counter}")
    counter += 1
Loop While counter <= 5            ' runs 5 times: 1, 2, 3, 4, 5

' Even if condition is false from the start, body runs once
Dim x = 100
Do
    lblDemo.Text = "This always runs!"   ' executes once
    x += 1
Loop While x < 5                  ' False immediately, but body ran once

' Guess the number game -- must try at least once
Dim secret = 42
Dim attempts = 0
Dim guess As Integer
Do
    Dim input = InputBox($"Guess the number (attempt {attempts + 1}):")
    Integer.TryParse(input, guess)
    attempts += 1
    If guess < secret Then MsgBox("Too low!")
    If guess > secret Then MsgBox("Too high!")
Loop Until guess = secret OrElse attempts >= 10
lblResult.Text = $"Got it in {attempts} tries!"

' Exit For: stop as soon as we find what we need
Dim names() = {"Alice", "Bob", "Carol", "Diana"}
Dim found = -1
For i As Integer = 0 To names.Length - 1
    If names(i) = "Carol" Then
        found = i
        Exit For             ' stop searching
    End If
Next
lblResult.Text = If(found >= 0, $"Found at index {found}", "Not found")

' Continue For: skip even numbers, only process odds
Dim sb As New System.Text.StringBuilder
For i As Integer = 1 To 20
    If i Mod 2 = 0 Then Continue For   ' skip evens
    sb.Append(i & " ")
Next
lblOdds.Text = sb.ToString()         ' "1 3 5 7 9 11 13 15 17 19 "

' Exit Do: break when a prime is found
Dim start = 50
Do While start < 1000
    If IsPrime(start) Then
        lblPrime.Text = $"First prime >= 50: {start}"
        Exit Do
    End If
    start += 1
Loop

' Multiplication table (3x3)
Dim sb As New System.Text.StringBuilder
For row As Integer = 1 To 10
    For col As Integer = 1 To 10
        sb.Append((row * col).ToString().PadLeft(4))
    Next
    sb.AppendLine()      ' new row
Next
txtTable.Text = sb.ToString()

' Fill 2D array
Dim grid(4, 4) As Integer
For r As Integer = 0 To 4
    For c As Integer = 0 To 4
        grid(r, c) = r * 5 + c
    Next
Next

' Bubble sort (classic nested loop algorithm)
Dim arr() As Integer = {64, 34, 25, 12, 22, 11, 90}
For i As Integer = 0 To arr.Length - 2
    For j As Integer = 0 To arr.Length - 2 - i
        If arr(j) > arr(j + 1) Then
            Dim temp = arr(j)
            arr(j)     = arr(j + 1)
            arr(j + 1) = temp
        End If
    Next
Next
lblSorted.Text = String.Join(", ", arr)   ' "11, 12, 22, 25, 34, 64, 90"

Private Sub btnFib_Click(sender As Object, e As EventArgs) Handles btnFib.Click
    Dim n As Integer
    If Not Integer.TryParse(txtN.Text, n) OrElse n < 1 OrElse n > 50 Then
        lblResult.Text = "Enter N between 1 and 50." : Return
    End If

    Dim a = 0
    Dim b = 1
    Dim sb As New System.Text.StringBuilder
    sb.Append("0, 1")

    For i As Integer = 3 To n
        Dim next = a + b
        sb.Append($", {next}")
        a = b
        b = next
    Next
    lblResult.Text = sb.ToString()
End Sub

' Find all primes up to N using trial division
Private Function IsPrime(n As Integer) As Boolean
    If n < 2 Then Return False
    If n = 2 Then Return True
    If n Mod 2 = 0 Then Return False
    Dim i = 3
    Do While i * i <= n
        If n Mod i = 0 Then Return False
        i += 2
    Loop
    Return True
End Function

Private Sub btnPrimes_Click(sender As Object, e As EventArgs) Handles btnPrimes.Click
    Dim limit As Integer
    If Not Integer.TryParse(txtLimit.Text, limit) Then Return

    lstPrimes.Items.Clear()
    For i As Integer = 2 To limit
        If IsPrime(i) Then lstPrimes.Items.Add(i)
    Next
    lblCount.Text = $"{lstPrimes.Items.Count} primes found"
End Sub